Reduction of alkyl nitroaromatic compounds



Patented Feb. 23, 1 954 REDUCTION F ALKYL NITROAROMATIC 1 COMPOUNDS Ralph B. Thompson, Hinsdale, 111., assignor to Universal Oil Products Company, Chicago, 111., a corporation of Delaware No Drawing. Application August 30, 1951, Serial No. 244,463

11 Claims. 1

This invention relates to a continuous process for reduction of certain alkyl nitroaromatic compounds to form the corresponding amino-sub: stituted alkylaryl derivatives, the latter having utility as intermediates in the production of a specific class of oxidation inhibitors for organic compounds and as fuel oil additives in which the additive has the capacity to reduce or eliminate the tendency of the fuel oil to undergo storage deterioration. More specifically, the invention concerns a process for efiecting hydrogenation of the indicated class of nitro compounds in the presence of a particular mixture of solvents which enables the process to be operated on a continuous basis accompanied by high yields of the desired amino product.

' It is one object of the present invention to provide a process for the catalytic. reduction of a nitroaromatic compound whereby the reduction may be effected continuously with an accompanying high yield of conversion product. Another object of the invention is to provide a solvent for alkyl nitroaromatic compounds which maintains the compound and its reduction product in homo 'geneous solution during the reductive hydrogena- In one of its embodiments the present invention concerns a process for selectively reducing the nitro group of an alkyl nitroaromatic compound to its corresponding amino-substituted analog which comprises hydrogenating an alkyl nitroaromaticcompound dissolved in a solvent comprising a paraftinic hydrocarbon which is liquid at the ambient pressure and temperature conditions and an alcohol selected from methanol and ethanol in the presence of hydrogen and a metallic hydrogenation catalyst at hydrogenation reaction conditions.

A more specific embodiment of the invention concerns a process for reducing the nitro group of an alkyl nitrobenzene in which the alkyl substituent contains at least 7 carbon atoms per alkyl groupto the corresponding alkyl aminobenzene which comprises passing said alkyl nitrobenzene contained in a liquid solvent comprising a mixture of from about 10 to about 50% by weight of methanol and a C 6 parafflnic hydrocarbon over a supported. metallic hydrogenation catalyst at hydrogenating reaction conditions. v

Other objects and embodiments of the ,invention relating to specific aspects thereof will be referred to in greater detail in the following further description of the invention.

Certain members of the series of compounds characterized as alkyl aromatic amines, diamines and N -alky1 aromatic monoand polyamines are uniquely useful as intermediates in the manufacture of oxidation inhibitors, compounds characterized as being effective in relatively small concentrations to inhibit oxidative deterioration of organic compounds and to reduce or eliminate the tendency of oils, particularly petroleum-derived fuel oils, to undergo certain types of deterioration, such as sludge formation, discoloration, etc., which occurs upon storage of such oils.

The additives are prepared from such amines as intermediates by conversion to their N-alkyl substituted derivatives, by conversion to their hydroxy-substituted derivatives, by conversion to their guanidine and biguanide derivatives, etc. The resulting products are customarily employed as additives to organic compositions subject to undesirable changes due to oxidation and/or storage to thereby reduce the economic loss occasioned by such deterioration. The present amines are also useful as intermediates in the preparation of pharmaceuticals, insecticides, detergents, and for many other uses which will readily be recognized by those skilled in these arts. Commonly recognized practice in the synthesis of these compounds is to utilize alkyl aromatic amines as starting materials, into the molecular structure of which are introduced the substituent radicals, if any, essential for obtaining the final product having the selective property desired.

One of the specific problems associated with the initial synthesis of the alkyl aromatic amine by reduction of the corresponding alkyl nitroaromatic compound is the problem of finding a suitable solvent in which the nitro compound is soluble and which does not interfere with the consummate reduction of the nitro group with hydrogen. Although individual solvents are known which dissolve completely the alkyl nitroaromatic compound, such as a liquid aliphatic hydrocarbon, the solvent may otherwise be unsuitable for this purpose because of the resistance of the nitro groupto undergo reduction when dissolved in such a solvent, or because the reduction efiected in such a solvent proceeds slowly, necessitating extended periods of reaction and eventual-separation of amine product from unreduced alkylnitroaromatic feed stock by distillation or other means. Of the organic compounds commonly utilized as solvents, such as the ketones, alcohols,

some cases the nitro group fails to undergo hydrogenation in the presence .;of such relatively high molecular weight alcohols except at considerably higher reaction temperatures and at such temperature levels, a large proportion of 1 the amine product is converted tourldcsirabletars;

sludges and other degradation products. The low molecular weight alcohols, suchas methanoliand ethanol, on the other hand, do not dissolye.v the charging stock in sufficient concentration to provide a single-phase solution in the concentration required for continuous-operation and are generally-unsuitable as solvents for the-reduction process. It has been-discovered that .a-combina tion of at least two solvents-comprising a mixture of the li pes; referred to as liquid parafiinic hydrocarbons and low "molecular Weight 1 alcohols, specific-aily,--methanol and ethanol, preferably in mixtures which the alcohol is present to the extent ot from-- about to -about150% by weight oi the mixture, results in v asingle-phase solution bfthecharging stock and provides-a satisfactory solvent in which-the hydrogenation or reduction of theallgyl-nitroaromaticcompound to the correspending-amine proceeds substantially to completion and-results-in a high yield of the product within a reaction time suitable for continuous operation of the reduction process.

' suitable alkyl' nit-roaromatic compounds utilizable as initial charging stocks in the present reduction process may be characterized as-members oi either the =mononuclear class (if-aromatics (that is, the-*benzenoid' series) --or the polycyclic aromatic compounds containing at least one-alkyl substituent --having atleast -7- carbon atoms per alhyl'group in the benzenoid series oi-compou-nds andat -least l-alky-l group of at least-4- carbon atoms in-chainlength in the polycyclic aromatic series. "The-nitro compounds-suitable as charging-stocks herein may contain one or more nitro substituentsand maybe additional-ly-substituted by suc-h groups as hydroxy, alkoxy, acyloxy, halogen sul-fo or other-non-interfieringsubstituentswhieh-are-not in themselves independently oapableof reductionby hydrogen under the conditionsemployed inthe present: process or which do'not interferewith thereduci-ng capacity of the present metallic hydrogenation catalyst. "The aromatic nucleus of the charging stock may additionallycontain other alkyl groups ofany'des-ired chain length. oraryl, cycloalkyl, arallgyl substituentsp-provided such additional radicals'do not result in hindrance of 1 the nitro group undergoing reductive hydrogenation. Typical alkyl aromatic nitro compounds utilizable as charging stocks in the present process include such-repre sentative compounds as ortho-, meta-, paraheptylnitrobenzene, 4-nitro-2-octyl toluene, 4- nitro dodecylbenzene, 4,7-dinitro-3-butyl naphthalene, 4,4 dinitro- 3 decyldiphenyl, 3,5 dichloro-e-nitrohexadecylbenzene, and other-compounds-within the.- general qualifications hereinahove specified. The products'for-med by the reduction-.arenamed according to the amine corresponding thereto; for example, dodecylnitrobenzone. omreduction in accordance with the present *.-,-4=. process, produces dodecylaniline. The compounds containing a benzenoid aromatic nucleus are particularly adapted to use in the present process as charging stock therein for the ultimate production of inhibitors and additives, although the process is not necessarily limited thereto.

The mixture of liquids'comprising Ithe selective solvent for the alkyl nitroaromatic compound charged into the hydrogenation reaction of the present process comprises a combination of methanolor ethanokwithe liquid paramnic hydrocar- 'bon containing at least 4 carbon atoms per molecule of either aliphatic or cyclic paraffinic structure. "The; preferred paraffinic solvents for this 5 purpose are'the-parafiins of intermediate molecular weight containing from about 5 to about 8 .carbon atoms pen-molecule, such as hexane, cyclohexane, pentane, n-heptane, and mixtures thereof, such .as a Ca fraction of a natural gasoline containing various pentane, hexane, heptane and octane isomers. The hydrocarbon portion of the solvent may also contain minor'amounts of olefimc and/or cycloolefinic' hydrocarbons which are ordinarily hydrogenated under the presentconditions of operation and in the presence of 'i the metallic hydrogenationcatalyst to form thetcorresponding parafiinic hydrocarbons. The combination solvent; preferably contains' from about 10 to' about"56% by weightof thealcoholic component, that is; the methanol or. ethanol portion of the mixed, solvent. :Theialkyl nitroaromatic compound may be charged to the hydrogenation. reactor dissolvedin the specified solvent mixture inany concentration from a relatively dilute'solution .containingfrom about 0.1 part-of the nitro compound per parts, of-solu'tioni to relatively concentrated solutions of the chargestock in the solvent, up to about 50 pa'rtsperlOO, parts lay-weight of solution.

Iheselective reduction of ,the nitrogroup .of the allgyl nitroaromaticcompound charged. to the reaction of the present, process is effected, in,.the pretence, of ametallic hydrogenation catalyst, preferably distributed .upon a refractorysupport, such as .a, porous metaLoxide. Suitable. metallic hydrogenation catalysts whichefiect the present. reduction to the .correspondingamino. radicals are, for example, theirongroupmetalshofthe elements of group VIII of the periodic tablapref erably the metals: .nickel,.cobalt, and .particularly the, nobleimetalsoi the .groupNIII elements: latinum and palladium. Asiindicated, :these metals are preferably-.ldclmsited.upon .a carriertherefor which, provides anactive cataIyticsurface exposed to 1 they solution of the alkyl mm. aromatic compound reactantfin the. solvent. andv in the presence 'of gaseous "hydrogen. Typical inertsupports-for the active'catalytic metals are such-materials 'as'alumina, either as the naturally occurring bauxite eras a synthetically; prepared xlehydrated =-alum-i-num hydroxide, silica, porous carbon, such as wood charcoal,--coal,-coke, etc.,- naturally occurring porous minerallities; such as kieselguhr; EuilersEarth,- Bentonite, -andother natural'clays and porous substances derived from mineral origin. 'The-metho'dfor preparing such metallic hydrogenation-catalysts a welhknown procedure in this "art, "but, i in general,- the catalyst is eithersupplied to the hydrogenation =zone the form of a flocculentffibrous, or other p0 rous-or fragmentary structure. "The; metallic catalyst may be composited withthe support in any suitable manner, such -asfor example, *by'iprecipitating' the metallic hydroxide orr the'supportr ing material and thereafter reducing the oxide to the metallic state. Supported catalysts for hydrogenation purposes generally contain from about 0.1 to about 50% by weight of the metallic component composited "with or deposited onthe supporting material.

Hydrogenation or reduction of the present alkyl nitroaromatic compounds utilized as charging stock herein is effected by passing the. dissolved charging stock in the combination solvent herein provided over the catalyst or in contact therewith in the presence of hydrogen at a ternperature of from about 30 to about 200 C. and at apressure at least suflicient to maintain liquid phase in the hydrogenation zone, prefer-'- ably at a superatmospheric pressure of from about to about 100 atmospheres and for a period of contact of the charging-stock with the catalyst of from about minutes to about 2 hours. The contact of the reactants with the catalyst may be accomplished in either a batch-type reactor or by continuous means, for example, by flowing the liquid reactants containing the dissolved alkyl nitroaromati-c. compound over orthrough the catalyst in contact with the hydrogen until substantially complete reduction of the nitrogroup is effected. In general, the present catalytic operation wherein the charging stock is dissolved in the combination solvent is particularly adapted to a continuous type of operation, since the solvent herein provided produces a one-phase solution of the charging stock'in the solvent capable of contacting the catalyst as a homogeneous mixture. The catalyst, for example, may be depos'- ited upon an inert carrier and placed in a vertical tubular reactor through which the charging stock, dissolved in the present solvent and in contact with hydrogen at the desired temperature and pressure reaction conditions, is continuously forced through the bed of catalyst in either a downward or upward direction of flow and under conditions which provide a residence time in the tubular reactor and in contact with the catalyst sufficient to accomplish substantially complete reduction of the nitro group or groups on the alkyl nitroaromatic compound. The product stream issuing from the reactor may be charged directly into a distillation unit to recover the solvent components which may be recycled to the stage in the process flow in which the charging stock and solvent are mixed to provide the'single phase solution charged into the hydrogenationreactor.v Separation of the amine from unchanged charge is usually not possible by extraction with'an aqueous acid phase, since the amine salts with relatively large alkyl groups of 8 carbon atoms ormore are not soluble in water. Therefore, it is'desirable to efiect complete reduction of the charge'stock in one pass through the reactor, since in this case' the solvent and water of reaction may be removed by distillation, leaving as residue the amine product which is in; a suitablecondition and of suflicient'purity forsubsequent reactions involving the amine. "I'h'e present invention will be further illustrated with :res'pect to specific embodiments thereof in the following examples which are intended merely .to' illustrate andnot to limit the generally broad scope of the invention in strict accordance therewith.

"A series of p-nitro alkylbenzenes in which the alkyl groups contain from 7 to 12 carbon atoms per'alkyl group'were mixed with various solvents to determinetheir solubility therein and for those solventscontaining at least 5% of the alky1nitrobenzene component, the resulting mixture together with hydrogen at 1000 pounds per square inch pressure was chargedat -a space velocity (volumes of liquid charging stock per volume of catalyst perhour) ofv from 0.1 to about 1.0, into the top of a vertical catalyst tube containing a hydrogenation catalyst consisting of 0.3% platinum deposited on alumina: The catalyst in the tubular reactor was maintained at an average temperature (an average of the tempera ture at the top, middle, and bottom of the tube) of from about to about 200 C. For those solvents which fail to dissolve at least 5% by weight of the charging stock, the solvent was rejected as unsuitable for continuous hydrogenation at the hydrogenating conditions specified above. The following table presents the operating data utilized in the reduction, including the per cent conversion of the alkyl nitrobenzene charging stock to the corresponding amino substituted compound.

TABLE Conversion of alkyl nitroaromatic compounds to amines by hydrogenation in various solvents Percent Cha I stock S l t Space 'Iempera- C Nitro rgmg 0 Van Velocity 3 3 28,

verted P-Nitrododecylbenzene Hexane-Heptane 0. 20 107 13 D0 r10 0.50 54 Do do 0. 20 228 1 0. 9 Dn Mefhanni Do Ethane] a a Do Propanol 0.20 150 87 D Ethy 0. 20 150 05 Do 10% Methanol-90% Hexane- 0.20 1.2 Do 40% Methanol-60% Hexane. 0.20 143 0.03 Do 407 Ethanol-60% Hexane. 0. 20 143 6. 2 Do-.. 40% Methanol-60% Hexane- 0.23 0.3 Do -do 0. 25 145 0. 2 p-Heptyidodecyibenzene Methanol (g 0) Do Ethylnnafnfn a z 3-Nitromethylnaphthaiene 40% Methanol-60% Hexane- 0.20 125 0.2

f The percent nitro compound remaining in the product as unconverted charging stock was determined by reduction with titanous chloride and back titration with ferric alum.

1 The color of the product in this run was very dark and a considerable proportion of the charging stock was gamer-ted to a tarry residue separated from the reaction mixture by distillation.

Rejec It is evident from theabove results that either an" alcoholsaione :ofiahydrooar-bon solvent alone unsuitable? toruuse as :=.a,:sq1.v,-ent;;in the-Loon tinuous reduction cramaikuinitroaromatiorcoma pound to; formtthe ccorresnflndingiiamine. 9A1? through ::a hydrocaroon solventi-aakonei mexane, eg. with, psnitrododecykhenzenel .sdiss olvesz the. charging stock. the: reduction:. converts-.0111 a portion of theaalmilmtroammaticitozamineguns less temperatures nhigh: enoughr mi;cause -.;othet undesflamareacti'ons; sue'lr: asfdeauunation. ar used. :Atthe ftemperaturewtarry;materials are: formedwhich causes ltheiamine to; be. highly impure and unsuitable :for; subsequent: reactions.

I ciaimwasmylinventionz 1. In; :a aprocess for seie'c'tively reducing Jithe nitro: r up: of ran i aikyk nitmaromatio; compound to th co r ondin emine-su stitu eo ethy aromatic compound, by contact Qflfllflfifit-mtmgd m poundwitna metallic-hydrogenation cata: yst n e. p e enceof'hydms n ano at lm drogenating conditions 0: temperature and pressure; the improvement whiolg'comprises passing over the catalyst a SOIUti OH Df said amt-mmaromatic compound in uaxmixture bf =.&. paraifinic hydrocarbon which is liquid at" said? conditions and. an coho sel c ed iro th exam: con s ingofmethanol and ethanol.

' 2, The. ocess iclaim'l manor charact ri ed in at said, mix ure ontains tromahout 10 to about 50% by'we 'ght o1" said-alcohol,

' i 3. The processor claim' 1 further characterizedin that said paraflinigi -hydrocarbonis aCa parafiim' -4. The process of "claim 1 further characterized inthat said .parafffinic hydrocarbon is a; mixtureof. Ce: and fil'rsparaflinic hydrocarbons.

5.- Thmprocessmf; claim: 1- fm the zcharacter' V 8 ized in that said alkylEhitroaromatic compound isamononuolean compound.

; '6. .Thepmcess: of. claim; further .oh rac rw ized: that; thezalkyl group'iofi said;.o.ornpo.und contains-at. least. 'Marbqn atoms.

' The prooess of :claim: further characterized in that said solution is .passedmver; saiohr: drogenatiqn: catalyst ..at.:,a:. temperature of irom about 1 6.0? to; about. 1250*.6. sand-.1- at, a superatr mospheriopressure.

w 8. In a process: for. .therproduction of. .dodecyl: aniline. by contacting dodecylnitrobenzeneiwith a metaliiohydrogenatiqn:ca12a1yst;m.the:presence of hydrogen andlat- .ihydrogenating. conditionswi temperature and pressure, the improvement which: comprises passingover the-.catalystma aolution: of; said dqdeeyipitrobenzene .in...a mix? ture of aparamnio; hydrocarbon whichis liquid at said: eonditionsi and. amalcohfllr: selected from the roup consisting of methanor and ethanol.

. The-pro ess ofclaimfi; further character? -iaed. that said parafiinie hydrocarbon a .CB Piilfifiin- 1 l0! Them-Quests of claim-8 further character-.- ized that. said temperature-11s about-60- to.- about;250- C. and said;pressure.v superat, mosnherie.

.1.The. 'm'ocessoi .e1aim:8.;fu1:ther character: ized; in that. said: mixture; contains: -from.;.about .0. 119. Q-bQut-fif; by weightaof. sai zalcohol.

IEEIQMBSQN.

I :Beierenees Cites menfilaei-th so ent Wernerz industrial and-"Eng; Ghemfi-vol 49, #9. (41-948)v pp. 157614). 

1. IN A PROCESS FOR SELECTIVELY REDUCING THE NITRO GROUP OF AN ALKYL NITROAROMATIC COMPOUND TO THE CORRESPONDING AMINO-SUBSTITUTED ALKYLAROMATIC COMPOUND BY CONTACT OF THE FIRST-NAMED COMPOUND WITH A METALLIC HYDROGENATION CATALYST IN THE PRESENCE OF HYDROGEN AND AT HYDROGENATING CONDITIONS OF TEMPERATURE AND PRESSURE, THE IMPROVEMENT WHICH COMPRISES PASSING OVER THE CATALYST A SOLUTION OF SAID ALKYL NITROAROMATIC COMPOUND IN A MIXTURE OF A PARAFFINIC HYDROCARBON WHICH IS LIQUID AT SAID CONDITIONS AND AN ALCOHOL SELECTED FROM THE GROUP CONSISTING OF METHANOL AND ETHANOL. 